Micro-Fabrication
Micro-fabrication techniques are used to process surfaces and bulk materials for applications in microelectronics, optics, bio-sensing, metrology, displays and life sciences, for example. By way of such techniques, the spatially-controlled/selective removal of materials from a surface can be done. This is achieved by using techniques such as, for example, those involving etching or, alternatively, involving the local deposition of materials on a surface. Etching and deposition of materials can be done using reagents dissolved in a chemical bath, commonly referred to as wet chemistry, or what are commonly referred to as dry processes where the reagents are in gaseous form.
For the selective removal of materials from a surface, those areas where it is desired for material to remain should be protected. It has been proposed to do such protection by using lithography where an organic resist or metal layer is patterned or locally deposited on a surface. Regarding the use of resists, two techniques are typically used for their patterning. The first is electron-beam lithography where an electron beam writer is used to expose a resist with a focused beam of electrons. Exposure of the resist to electrons modifies the solubility of the resist in some solutions, which are used to transfer the electron-beam written pattern into the resist. This step is called the development step and typically the resist is made more soluble in the areas exposed to the electron beam. The second typically-used technique is photolithography, which relies on the chemical modification of a photosensitive layer by light to remove, i.e. by the use of positive tone photoresist, or prevent the removal of the photoresist, i.e. by the use of negative tone photoresist, in the areas of light exposure during a development step. Patterning of resist materials is done with several processing steps such as spin coating, baking, exposure to electrons or light, developing the exposed resist, plasma cleaning steps, and metrology of the developed resist, for example. During these steps, expensive materials such as, for example, high-purity chemicals, solvents and metals, and instruments such as, for example, mask aligners, electron-beam writers, evaporators, are used. Waste materials and chemicals are produced, which have associated safety and environmental concerns for their disposal.
After the resist has been patterned, it is used as a physical barrier against chemicals from a bath or in the gas phase to protect the underlying surface from etching. Surfaces to be processed can be large in area, for example, greater than 1 m2, thus using proportional amounts of chemicals and resist—the size of such surfaces and the amount(s) of reagents used in their processing is, of course, translated into the size of the chemical baths that are used for such purposes. For such surfaces, the cost of processing equipment rises exponentially.
From the discussion above, it can be seen that micro-fabrication processes are typically based on several deposition, patterning, etching steps. For example, thin-film transistor arrays for flat panel displays are typically processed with between 30 to 60 processing steps and hard-disk drive heads may use over 250 steps, for example. As devices progress in the process flow, they become more valuable, yields diminish, and the materials and structures present on the surfaces of the devices become increasingly heterogeneous. These surfaces then have varying degrees of reactivity with chemicals in baths or in a gas phase. A layer, deposited or processed early in a process might be corroded or adversely altered in a subsequent step. Obviating such a problem may require substantial efforts for devising a processing chemistry that is specific to one type of material. If such chemistry cannot be found or implemented, fragile parts of a device must be specifically protected and additional processing steps have to be added for such a purpose. This adds to the overall cost for processing surfaces and causes reduced fabrication yields.
Wet-Etching
The transferal of a resist pattern into an underlying substrate can be done using wet chemistry, in particular, wet-etching. For this purpose, corrosive chemicals are typically used to remove material/atoms from the regions of a surface, which are not protected by the resist. Etch baths are typically large in volume, for example, they can be greater than 40 litres, contain toxic or hazardous chemicals, should be of well-defined composition, and are usually expensive because the chemicals which they contain are of a high chemical grade and the baths should be free of contaminating particulates. In some cases, such baths have limited stability and/or cannot be reused. They might have to be stirred, maintained at a specific temperature, de-aerated or a combination thereof. Reusing or disposing of etch baths is expensive. Flammable etch baths should be equipped with fire suppressing equipment depending on their volume. The CRC Handbook of Metal Etchants; Walker, P., Tarn, W. H., Eds.; CRC Press: Boca Raton, Fla., 1991, discloses that most of the common etch chemistries involve concentrated acids, combinations of acids, heated etch baths, stirring baths, toxic compounds, and/or alkaline solutions.
Baths may be sprayed and reagents may be recycled in processing tools for alleviating the consumption and waste of chemicals. Such processing tools reuse some of the reagents, the number of times of the reuse being dependent on the chemistry or purity degradation characteristics of the reagents. Such tools are typically expensive. Despite the availability of such processing tools, some micro-fabrication processes are sensitive to the history of chemicals and so reagents may not be reused. Also to be considered is that particulates tend to form and accumulate in baths, for example. Unstable baths have to typically be prepared shortly before use. Electroless deposition baths for silver (Ag) are, for example, mixed and sprayed directly onto substrates.
An introduction to micro-fabrication can be found in “Fundamentals of Micro-fabrication” by M. J. Madou, CRC Press, New York, 2002. Current reviews on micro-fabrication techniques have been given by Xia et. al in Chem. Rev. 1999, volume 99, pages 1823 to 1848 and Geissler et. al in Advanced Materials, volume 16, 2004, pages 1249 to 1269. Some of the techniques described therein provide remedies to one or several of the above-described issues by, for example, patterning surfaces directly without using resists.
Local Processing
Local processing of surfaces, i.e. the processing of a localized region or regions on a surface, has been done using a variety of scanning techniques, for example, using scanning tunneling microscopy, atomic force microscopy, and dip-pen nanolithography. These techniques use a controlled probe-surface interaction and involve serial writing. They are expensive by virtue of the: precision positioning mechanisms; micro-fabricated probes, which have a limited lifetime, and the use of specialized software. They are complex because of the extensive work that is done to develop reliable process parameters. These techniques also require several steps to be performed, such as, for example, treating the surface to be processed with a resist, writing the pattern, developing the pattern, selectively structuring the surface, and removing the resist left.
Patterning of a surface may be done using an inkjet. In this case a resist or material of interest is deposited onto a surface. This technique is versatile but lacks resolution due to the surface tension of liquids, which prevents forming droplets having a diameter below approximately 70 micrometers. Uncontrolled spreading and evaporation of inks on a surface also pose drawbacks for this technique. Viscous inks can be difficult to dispense and inkjet nozzles can be damaged by corrosive chemicals.
Laser ablation can be used to process surfaces by removing materials but this technique is limited in resolution, throughput and uses expensive lasers. Focused ion beams can be used to remove or deposit materials from or onto surfaces but this technique involves expensive equipment, must be operated under vacuum conditions, and is serial and, therefore, may be considered time-inefficient for surface-processing purposes.
Micro-contact printing is a technique for structuring surfaces. It uses a patterned elastomer, which is replicated from a mold, and that is inked and placed in contact with a surface. In the regions of contact, some of the ink transfers to the printed surface. It is used most notably to pattern self-assembled monolayers on gold (Au), silver (Ag), copper (Cu) or palladium (Pd), which are used to protect the printed areas of the metal from etchants. Despite the fact that micro-contact printing can process different-dimensioned, planar, and even curved substrates with increased accuracy compared to previously-proposed techniques, it relies on transferring the printed pattern into a surface in processing baths. Therefore, similar limitations as above-described with reference to known lithographic techniques may be encountered when micro-contact printing is used for patterning surfaces.
Examples of micro-contact printing for the patterning of surfaces are given in: Applied Physics Letter, volume 63, pages 2002 to 2004 by Whitesides et. al., Nano Letters, volume 3, 2003, pages 1449 to 1453 by Reinhoudt et. al, and Advanced Materials, volume 17, 2005, pages 1361 to 1365 by Grzybowski et. al.
Techniques have been proposed for the processing of surfaces without the use of resists or wet-etching. Examples of such techniques have been disclosed in: Nano Letters, volume 5, 2005, pages 321 to 324 by Frechet et. al, Nano Letters, volume 3, 2003, pages 1639 to 1642 by Gheber et. al and in Nature Materials, volume 4, 2005, pages 622 to 628 by Juncker et. al. Drawbacks associated with these techniques include that, being based on scanning probe microscopy methods, precision positioning techniques/features and/or the use of micro-fabricated probes make them costly. Their serial nature makes these techniques time-inefficient.
Accordingly, it is desirable to provide a method for processing a surface that mitigates and/or obviates the drawbacks associated with known surface processing techniques.